Currently, there is an ever greater need by the market for voltage regulators with low voltage drop, that is, regulators which can operate correctly even if the voltage drop between the supply voltage and the regulated output voltage is a fraction of a volt. These linear voltage regulators with low voltage drop are required for various reasons. For example, they improve efficiency in both battery-operated electronic systems and those operating with a mains supply. A regulator which supplies an output voltage of 5 V and needs a voltage drop of 5 V has an efficiency of 50% whereas, if it requires a voltage drop of only 0.5 V between the input and the output, its efficiency is more than 90%.
A reduction in the power dissipated by the regulator avoids the use of large dissipaters and enables less expensive housings to be used. A regulator which requires a voltage drop of 5 V when it is supplying a current of 1 A to the load has to dissipate a power of 5 W. In contrast, with a voltage drop of 0.5 V, it has to dissipate only 0.5 W. The reduction in the dimensions of the dissipater, or its elimination, and the reduction in the dimensions of the transformer (in mains applications) also permits a considerable saving of space.
The continual reduction of the supply voltages of electronic devices with the consequent spread of systems with a mixed 5 V and 3.3 V supply (the latter can be produced from the former simply by a regulator with a low voltage drop) necessitates the use of regulators of this type. Moreover, these regulators supply a constant voltage to the load even in motor vehicle applications in which the voltage supplied by the battery may fluctuate considerably because of changes in temperature or in the load currents. An example is the starting of the motor vehicle at low temperature, during which the battery voltage may fall to values slightly more than 5 V.
The element around which a voltage regulator is constructed may be a bipolar transistor or a MOS power transistor. In the first case, the minimum voltage drop is given by the saturation voltage V.sub.sat of the transistor. In the second case, the minimum voltage drop between input and output is related to the voltage Vgs supplied between the gate and source terminals and to the physical size of the transistor, and the voltage drop could thus be reduced even to a few tens of millivolts. Another advantage of MOS transistors, for example, those of DMOS type, is the smaller area of silicon occupied.
However, problems arise if it is attempted to produce a wholly integrated regulator which minimizes or reduces to zero the number of external components necessary for the regulator to be functional and stable and to have a rapid response to changes in the voltage regulated, with performance comparable to or better than normal regulators without a low drop. One of the main problems is that the gate voltage of the MOS transistor has to be brought to high values, usually to a voltage greater than the supply voltage.
Approaches according to the prior art use a charge pump to generate a voltage high enough to be able to drive the MOS power transistor. A circuit of this type is shown in FIG. 1. The voltage-regulator circuit shown uses a charge pump CP which supplies a voltage greater than that provided at an input IN of the voltage regulator. This voltage supplied by the charge pump CP supplies an output stage BUF of an error amplifier ERA which in turn controls a gate terminal of a power transistor PT.
The other main terminals of the voltage regulator are also indicated in FIG. 1. Thus, the output terminal OUT, the ground terminal GND and the adjustment terminal ADJ can be seen. As can be noted, the control loop of the voltage regulator is conventional, the non-inverting and inverting inputs of the error amplifier ERA being connected to a band-gap voltage reference BG and to the adjustment terminal ADJ, respectively. The drawing also shows a fold-back protection circuit FB. The other parts of the circuit of FIG. 1 are not described since they are not relevant for the purposes of the present invention.